Numerical control system for a lathe

ABSTRACT

A continuous-path numerical system controls the position of the cutter of a lathe with respect to the workpiece and the rate of rotation of the work spindle in accordance with a series of numerical commands encoded on a punched tape. Each command, in addition to position information, includes information relating to the desired rate of movement of the cutter over the workpiece in surface feet per minute and the desired rate of advancement of the cutter in terms of inches per revolution. The commands controlling the position of the cutter with respect to the center line of the spindle are continuously monitored and the quantity representative of the commanded separation is divided into the programmed surface feet-per-minute number to derive a number used to control the spindle motor drive. This system additionally calculates a feedrate number based on the incremental distances of motion along the X and Z axes called for by a particular command and the inches per revolution number. A pulse train generated by a transducer which senses the rate of rotation of the spindle is multiplied by this feedrate number to derive command pulse trains for DDA-type interpolators which generate control signals for the X and Z axes.

United States Patent 1 Cutler [54] NUMERICAL CONTROL SYSTEM FOR A LATHE[75] Inventor: Hymie Cutler, Detroit, Mich.

[73] Assignee: The Bendix Corporation, Southfied,

Mich.

[221 Filed: Aug. 5, 1970 [21] Appl. No.: 62,231

[52] US. Cl. ..235/151.1l, 318/569, 318/571, 318/573, 318/39 [51] Int.Cl ..G06i 15/46 [58] Field of Search... ..235/l51.l l; 318/567-574, 600,603, 35, 39, 51

[56] References Cited UNITED STATES PATENTS 3,573,588 4/1971 Geyer et al..318/571 3,325,710 6/1967 Reynolds ..3l8/39 3,548,172 12/1970 Centneret al.... ....235/l5l.11 3,218,532 11/1965 Toscano ..3l8/569 X 3,191,2056/1965 Gilbert ..3l8/39 X 3,270,186 8/1966 Centner....' ....235/l51.113,418,549 12/1968 Emerson et a1... ..3l8/57l X 3,449,554 6/1969 Kelling..3l8/568 X Primary Examiner-Malcolm A. Morrison AssistantExaminer-Jerry Smith [4 1 Apr. 3, 1973 [57] ABSTRACT A continuous-pathnumerical system controls the position of the cutter of a lathe withrespect to the workpiece and the rate of rotation of the work spindle inaccordance with a series of numerical commands encoded on a punchedtape. Each command, in addition to position information, includesinformation relating to the desired rate of movement of the cutter overthe workpiece in surface feet per minute and the desired rate ofadvancement of the cutter in terms of inches per revolution. Thecommands controlling the position of the cutter with respect to thecenter line of the spindle are continuously monitored and the quantityrepresentative of the commanded separation is divided into theprogrammed surface feet-per-minute number to derive a number used tocontrol the spindle motor drive. This system additionally calculates afeedrate number based on the incremental distances of motion along the Xand Z axes called for by a particular command and the inches perrevolution number. A pulse train generated by a transducer which sensesthe rate of rotation of the spindle is multiplied by this feedratenumber to derive command pulse trains for DDA-type interpolators whichgenerate control signals for the X and Z axes.

Attorney-William F. Thornton, McGlynn, Reising, l5 Claims,6DrawingFigures Milton & Ethington and Plante, l-lartz, Smith & Thompson 5 r36DA SPINDLE CONV DRIVE f 58 s's FEED RATE 40 INTERPOLATOR I X-AXIS X-AXISINTERPOLATOR SERVO CUTTER 1 4 6 PROGRAMMED Z-AXIS Z-AX|S CONTROLLERINTERPOLATOR SERVO i I l l I 5-? d) TAPE 62 READER r RADIUS STOREPATENTEDAPR3 I973 7 5,551

SHEET 1 OF 2 TOOL PATH ON WORKPIECE SURFACE O O 0 0 O O G TOOL O O O O 5O O 2 O O O O O O F o o 2 0 o 0 CUTTING o o 0 SPEED SFM O O o O 5 2 O Oo O 9 19. o o o o 5 o o O o O o o o O x O O o 0 g O 0 O O 5 60 x 0 0 O O6 DA I SPINDLE o o 2 CONV Dmvg o o o .o 5 o o o 0 Z 58 /0 O o 8 FEEDRATE 0 o l INTERPOLATOR 0 O 2 O o O O 5 5 O 0 ES 5% ,4?

I ,42 50 X-AXIS I X-AXIS INTERPOLATOR SERVO IW'iCUTTEF? 1/ 4'8 J j?PROGRAMMED Z AXIS Z-AXIS CONTROLLER INTERPOLATOR SERVO I v =5 a) 621.\"\ /;.\"m/ TAPE READER Hymze Cali/er RADIUS Y STORE Z W! flmwATTORNEYS PATENIEIIIPII3 I973 SHEET 2 OF 2 r VARIABLE I GEAR g E SPEEDBOX MOTOR MARKER I I PULSE 53 PULSE I wa GENERATOR TRANsDuCER I we IMOTOR CEAR 90 I: SPEED RATIO MARKER I CONTROL CONTROL DETECTOR DECODEA9Q44 I I I 'M' I [x DIRECTION I ADD I OF MOTION --suBT SYNCHRONIZER I ["6RADIUS T 96 TSIOFIE m C liu "8/ PRELOAD ON 53 F 98 L 55;: 3 F I I 20.2 II DATA THREADINC I I 106 I INPUT I CONTROL MODE STORE I ADDER I T IOVERFLOW I I I l I l DETECTOR I FRN I I i [7? STORE I R-I |NE I I 1 I I-10? '54 I I PR COMPUTE I lag I I sTORE FRN 85 I G {48 I I I l I A X IIZQ' ADDER I L I STORE I I L OVERFLOW I I I :2 0 DETECTOR "T I I? K IR-LINE I l I E... A72 I I -I I sTORE ,72 J50 I I at? I 5PM COMPUTE I I ISTORE RPM 5i I I I 50 I I ADDER I I Ifi RANGE MAx GEAR I DETECTOR I IsTORE RANGE RPM I R-LINE I l I I I .1z I I I I I I INVliN'IUR. I HymieCailer BY I BACKGROUND OF THE INVENTION l. Field of the Invention Thisinvention relates to contouring numerical control systems for lathes,and more particularly to such systems wherein the input informationincludes information relating to the desired rate of motion of thecutter with respect to the workpiece surface and the rate of advancementof the cutter with respect to the spindle rotation. These inputs areused along with commanded increments of motion to derive control for thelathe spindle speed and the cutter motion velocity.

2. Prior Art Contouring-type numerical control systems for machine toolsoperate to control the motion of the machine cutter with respect to aworkpiece through a path which is defined by a sequence of motionsegments. These segments are encoded in numerical form on a storagemedia such as a punched tape as a series of command blocks, each ofwhich calls for a particular segment of motion of the controlled machineparts along a straight line or a circular or parabolic arc. Each blockof information defines the nature of a path segment, and generally itsends points, in terms of coordinates along at least two mutuallyperpendicular axes. The system includes a servo drive for eachcontrolled axis and means for generating appropriate control signals forthe drives from the input numerical data, so that the controlled partsare moved through the prescribed path.

In addition to controlling the path that the cutter takes with respectto the workpiece, it is necessary to achieve control over the rate ofmotion of the cutter so that the operation is performed in the minimumtime consistent with avoiding damage to either the tool or the workpieceor excessive wear on the tool due to overheating. Conventionally thisrate 'control is achieved by encoding a feedrate number with each motionblock on the input tape. This number may be used to directly control therate of generation of the control signals, or it may be employed alongwith the commands for incremental motion along the various axes tocalculate a Feedrate Number" that is used to control the rate ofgeneration of the control signals. Additionally, in machines such aslathes or mills which have a rotary spindle, a command block contains anumber which controls the rate of rotation of that spindle. Thesefeedrate and spindle speed numbers are chosen by the programmer so thatthe machine will be operating in an optimum manner for the segment beingcut, taking into consideration all pertinent quality and economiccriteria.

Several problems are presented by this form of specification of thefeedrate and spindle speed numbers. First, the actual calculation of thenumbers is quite complicated so that in the case of all but the simplestparts, hand programming is sufficiently lengthy that off-line computerprocessing becomes a virtual necessity. Second, in order to maintainoptimum machine operation without varying spindle speed or feedratenumber during the formation of a particular segment it is necessary todivide the total motion into a large number of very small segments. Thislengthens the computation time and when the tape must be edited to debuga program or optimize feeds and speeds a the cutter over the workpiece.

large number of command blocks must be changed raising the possibilityof additional errors being introduced by this reprogramming.Additionally, the values of speed and feed included with the block arechosen on the assumption that the machine will be operating at thecommanded rates, but the spindle drive will generally respond veryslowly to changes in commanded speed so that the resultant operation may0 not be proceeding in the optimum manner.

In order to avoid these shortcomings of the system of designating thedesired feedrate and spindle speeds in a command block it has beensuggested that provision be made to modify programmed speeds and feedsas a function of the actual machine operation. In one scheme used on alathe, two potentiometers are provided which are driven by the motion ofthe transverse machine slide. One is used to attenuate theprogrammedspindle speed as the distance between the cutter and the workpiececenter of rotation increases, in order to maintain the rate of movementof the cutter over the tool at approximately a constant rate, and theother is used to attenuate the programmed feedrate. The result is thatas the radius of the cut increases, both the spindle speed and thefeedrate decrease. This provides a relatively constant rate of cuttermotion with respect to the workpiece and the chip-load. However, thissystem still does not correct for lags of spindle speed in response to acommanded change, and it requires additional mechanism. which is subjectto failure and drift of calibration.

SUMMARY OF THE INVENTION The present invention contemplates a system forcontrolling the rate of motion of the cutter of a lathe with respect tothe workpiece which greatly simplifies the part programming, eliminatesthe need for extra mechanism, and adjusts the control signals to theactual machine performance so that an optimum cutting operation isachieved.

Broadly, in a preferred embodiment of the invention the part programused with the system specifies, in addition to the geometry of thecommanded motion, the desired relative speed between the cutter and theworkpiece in surface feet per minute (SFM) as well as the inches of feedper spindle revolution in terms of the vector .distance of cutting-tooladvance per spindle revolution (IPR). These SFM and IPR only need to beprogrammed when they are to be changed in value. The controlcontinuously monitors the commands which control the distance betweenthe cutting tool and the center of rotation of the workpiece byalgebraically summing the command pulses associated with the motion ofthe cutter normal to the workpiece center line of rotation. Inalternative systems the motion of the cutter itself, rather than themotion commands, could be monitored by a suitable transducer to derivethis number. This dimension is equal to the radius of the workpiece atthe point of the cut. The control divides this radius figure into theprogrammed SFM to derive a control signal for the spindle drive motor.As the workpiece radius increases the spindle drive speed decreases inorder to achieve a constant value of the velocity of A transducerprovides output pulses for each increment of rotation of the workpiecespindle to generate a pulse train. This signal, proportional to spindlespeed,

could be derived from the spindle speed command signals, rather than atransducer, in alternative embodiments. Thislatter approach simplifiesprogramming and lowers hardware costs, but sacrifices control accuracysince it neglects differences between actual and commanded spindlespeed. In the preferred embodiment the transducer output pulse train ismultiplied by a feedrate number which the control calculates from theIPR number and the incremental motion commands for the two controlledaxes of the feedrate number. The output of the multiplier which acceptsthe spindlepulses and the feedrate number constitutes a pulse trainwhich is used to control the rate of operation of the interpolator whichcommands the X and Z motions on the basis of the encoded incrementalcommands. In this manner the commands are performed at such rates as toproduce a vector motion rate which is a direct function of both the rateof spindle rotation and the feedrate number so as to achieve a constantfeedrate in terms of inches per revolution Since the rate of productionof the lathes on which such numerical controls are employed is generallylimited by the capabilities of the cutting tool, programming a motion interms of SFM and IPR will achieve the maximum cutting rate possiblewithout excessive damage or tool wear. These SFM and IPR figures arerelatively constant from block to block and since they employ a minimumamount of calculation they substantially simplify programming and allowhand programming as opposed to computer programming with 'a broadvariety of parts. i

Other objects, advantages and applications of the present invention willbe made apparent by the following detailed description of a preferredembodiment of the invention. The description makes reference to theaccompanying drawings in which:

FIG. 1A is an illustration of a cutter and workpiece illustrating thedefinition of inches per revolution or IPR;

FIG. 1B is an illusii'ation of a second form of workpiece and cutterfurther illustrating the definition of IPR;

FIG. 2 is a diagram of a workpiece and cutter illustrating thedefinition of surface feet per minute or SFM;

FIG. 3 is a broad block diagram of a control system forming thepreferred embodiment of the present invention;

FIG. 4 is an illustration of a section of control tape containing atypical command block for control of the machine of the preferredembodiment; and

FIG. 5 is a more detailed block diagram illustrating the preferredembodiment of the invention.

As stated previously, the present invention is applicable to lathes andoperates to control the motion of the cutter of the lathe with respectto the workpiece so as to undergo motions commanded in input blocks atrates determined by further information encoded in the input blocks interms of [PR and SFM. FIGS. 1A and 1B illustrate the definition of IPRand FIG. 2 illustrates the definition of SFM. Y

' In FIG. 1A, the cutter is illustrated as forming a cylindrical cutalong a workpiece 12. The helical path 1 of the cutterwith respect tothe workpiece is illustrated by line 14 and the axial motion of thecutter as the workpiece undergoes 1 full revolution, i.e. the lead'ofthe cut, is equivalent to IPR. In FIG. 1B, the cutter 16 is illustratedas forming a conical cut on a workpiece 18 by a combination of motionparallel to the axis of revolution of the workpiece and motion normal tothat axis. IPR is again the lead of the helical cut formed but asillustrated in this figure the IPR is a vector distance measured interms of the resultant linear motion of the cutting tool. The force onthe tool cutting edge per unit depth of cut for a given workpiecematerial is proportional to the area of shear which is the product ofthe IPR and depth of cut. Thus by controlling the IPR, the cutting forceon the tool will be controlled.

In FIG. 2 a cutter 20 is shown as forming a cut on a workpiece 22 whichis viewed along its rotational axis. SFM is simply the rate of motion ofthe cutter over the workpiece surface in surface feet per minute. Theheat generated in a tool cutting edge for a given workpiece material,will be greatly influenced by the rate of abrasion of the workpiece overthe tool which is measured by SFM. Thus, by controlling the SFM, therate of the tool wear can be controlled.

The broad arrangement of a control system formed in accordance with thepresent invention is disclosed in FIG. 3. Input data constituting thenecessary path information for the desired operation of the system isencoded on a punched tape 30 which is sequentially provided to a tape,reader 32. The signals representing the output of the reader 32 areprovided to a programmed controller 34. The controller constitutes an appropriately programmed general purpose computer in the preferredembodiment of the invention. In alternate embodiments the controllermight be permanently wired. The operation of the controller 34 will bedescribed in functional terms in sufficient detail to allow a systemsanalyst, experienced in real-time systems, to design an appropriateprogram for any computer which will cause it to perform the desiredoperations, or to allow a skilled digital circuit designer to develophardwired apparatus for performing these functions.

In the preferred embodiment of the invention, a Micro Systems Model 810computer is programmed to perform the controller function. The operatinginstructions for this computer can be found in Micro 810 ComputerReference Manual," copyright 1969 by Micro Systems, Inc. of Santa Ana,Calif. This computer is in the mini class and larger and fastercomputers might be employed in the system at higherhardware cost.

The broad function of the controller is to receive the input informationfrom the tape reader 32, to operate upon that information in a mannerwhich will be subsequently described, and to provide suitable outputcontrol signals for devices which drive thecontrolled machine, generallyindicated at 36.

In the ,preferred embodiment of the invention the machine constitutes alathe which includes a spindle drive 38, operative to rotate a spindle40; and a cutter 42 which is movable normally to the axis of rotation ofthe spindle by X axis servo 44, and parallel to the. axis of rotation ofthe spindle by a Z axis servo 46. The controller 34 provides X commandnumbers to an X axis interpolator 48 and a Z axis interpolator 50. Theseinterpolators may be of the digital-differential-analyaer type disclosedin US. Pat. No. 3,128,374, and they operate to provide trains of outputpulses at rates proportional to their input numbers. The output pulsetrain from the X axis interpolator 48 is provided to the X axis servo 44while the command pulse train output of the Z axis interpolator 50 isprovided to the Z axis servo 46. The X and Z axis servos 44 and 46 maybe of the phaseanalog type described in US. Pat. No. 3,0l 1,110, whichprovide one increment of output motion for each pulse received.

The rate of generation of the output pulses by the interpolators 48 and50 is controlled by a feedrate interpolator 52, that generates a pulsetrain which is provided to both of the interpolators on line 54, andcontrols their rates of operation. The output pulse train on line 54 isgenerated by the feedrate interpolator, which may also be of the typedisclosed in US. Pat. No. 3,128,374, as a function of afeedrate numberprovided to the interpolator 52 from the programmed controller on line56, and a pulse train provided to the interpolator from the spindledrive 38 on line 58. The interpolator effectively multiplies the pulsetrain provided on line 58 by a fractional quantity represented by thenumber provided on line 56, to provide an output pulse train on line 54having fewer pulses than the input pulse train on line 58.

The pulse train on line 58 is generated by a transducer associated witha spindle drive which produces an output pulse for each incrementalrotation of the spindle. The rate of rotation of the spindle drive 38 iscontrolled by information provided from the controller 34 via adigitalto-analog converter 60.

The pulses provided by the X axis interpolator are also fed to a radiusstore unit 62 which is a reversible counter that effectively maintains avalue proportional to the normal distance of the cutter from theworkpiece axis of rotation, or the radius of the out being formed. Thisvalue is provided to the programmed controller 34 and is used by thecontroller to calculate the value of the control signal provided to thespindle drive.

The input information to the system on tape 30 is arranged as a sequenceof blocks, each commanding one increment of motion, and FIG. 4illustrates a typical block. The punched tape 30 is of the conventionaleight-channel variety and the tape reader 32 senses one line across thewidth of the tape at a time, with each line being representative of aparticular character in the coding system. Assuming that the tape movesinto the reader 32 in the upward direction, as viewed in FIG. 4, thefirst three lines are coded to represent G-S 2. This code indicates tothe controller that the feed information contained in the followingblock is encoded in terms of SFM and IPR, as opposed to moreconventional feedrate identificationsystems. The next four lines containthe codes for F 200. F is the code'symbol for the IPR number and thefigure 200 expresses the [PR rate in ten-thousands of an inch perrevolution. Thus, the [PR number is 0.02 inches per revolution. The nextfour lines decode as S 950. S is the SFM number in tenths of a foot perminute. Thus the SFM to actual physical or electrical positions withinthe controller.

Information provided to the controller 34 from the tape reader 32 isdecoded by a data input control section which directs the information toa number of stores, depending upon the address. The S number is providedto an SFM store 72; the F number to an lPR store 74; the delta X numberto store 76, and the delta Z number to a store 78. Other informationblocks may contain codings indicating the desired gear range of thespindle drive, and such information is provided to a gear range storeunit 80. All of this information is stored until replaced by equivalentinformation from subsequent blocks. The tape may also containinformation indicating that the machine is to be in the threading modeand this information is provided to the storage unit 82.

One of the functions performed by the controller 34 is to calculate afeedrate number which is provided to the feedrate interpolator 52 vialine 56. This computation is performed on the basis of the contents ofthe IPR store 74, the delta X store 76, and the delta Z store 78. Thecomputation is illustrated as being performed in a unit 84, but isactually performed by the computer components under program control atappropriate points in the computer's operational cycle and not at aspecific location within the system. The computation consists of solvingthe equation where K is a constant. This number is calculated for eachcommand block of information and is provided to a store unit 86 whichmaintains it during the time of operation of that block.

- The feedrate number used in connection with a pulse train derives froma transducer 90 which is connected to the machine spindle 40 by gearing92. The transducer 90 provides an output pulse for each increment ofrotation of the spindle. The transducer may be of the type disclosed inUS. Pat. No. 3,069,608 which has three brushes so that the direction ofthe rotation of the spindle may be interpreted from the occurrence ofpulses on these brushes. The decoder unit 94 performs thisinterpretation and cancels redundant pulses which may have occurred as aresult of vibration of the unit, and provides its output pulse train toa synchronizer 96 which brings the pulse timing into accord with theoverall rate of operation of the system.

The output of the synchronizer 96 passes through a gate 98 which isnormally open, and will later be discussed in detail, and is then usedto condition agate 100 which receives the feedrate store. Upon theoccurrence of each pulse in the input train to the gate 100, the numbercontainedwithin the feedrate store is provided to a serial adder 102contained within the feedrate number interpolator 52. This number isadded to a quantity contained in an R line 104, the contents of whichare continually recirculated through the adder 102. The occurrence ofoverflow pulses from this addition process is detected by unit 106 andused to generate a train of output pulses which act as add com mands forthe X and Z interpolators 48 and 50.

In connection with the X interpolator 48, the contents of the delta Xstore are provided to an adder 109, through a gate 108, each time apulse occurs on line 54 from the feedrate interpolator. The contents ofan R line 1 10 are continually recirculated through the adder 109 andoverflows from the addition process are detected by unit 112 andprovided to the X axis servo 44.

The motion of the cutter 42 in the X direction is thus controlled bythese pulses.

These X command pulses are also provided to a serial adder/subtractorunit 114 through which the contents of a radius store unit 116 areregularly recirculated. For each X command pulse that is provided to theunit 114 an addition or subtraction is made from the contents of theradius store unit 116 depending upon the sign of an input to the unit114 from the controller 34 which indicates the commanded direction ofmotion of the X servo. A number is manually preloaded into thev radiusstore at the beginning of the operation which indicates the normaldistance of the cutter nose from the axis of rotation of the spindle.This preset number is modified as X command pulses are generated inorder to maintain a value in the radius store 116 which is proportionalto the normal cutter distance, or the radius of a cut being formed bythe lathe tool. This signal is provided to the controller 34 via line118 and will be used in a manner to be described in order to control thespindle drive motor speed.

The Z command pulse train is derived by the interpolator 50 by addingthe contents of the delta Z store 78 into the contents of an R line 120,which is recirculated through an adder 122, each time an add commandpulse is received from line 54 via gate 124. An overflow detector 126generates the Z command pulses which are provided to the servo 46.

The system that controls the speed of rotation of the spindle utilizesthe. SFM number from the store 72 and the. radius, provided on line 118from the store 116. The system divides the SFM number by the radiusnumber and multiplies by the constant of 6 divided by 11', to provide anRPM signal which is stored in a location 130. This number is comparedwith the maximum spindle speed for the gear range stored in unit 80 bythe general purpose computer. This function is indicated by the box 132.

If the computed RPM is less than the maximum RPM, gate 134 is activated,which allows the calculated RPM to be used in the computation offthespindle speed number. If the calculated RPM exceeds the maximumobtainable RPM at that gear range, a gate 136 is activated, allowing themaximum RPM to be used in the calculation of the spindle speed number.The spindle speed number is equal to whichever RPM is provided by theunit 132, divided by the maximum RPM as derived from store 80. Theoutput of this calculation, which is identified in unit 138, is providedto the digital-to'analog converter 60 which generates an analog signalused by a motor speed control 140 to control the variable speed spindledrive 38.

The gear range store is also used to control appropriate solenoidscontained within a gear ratio control 142 to shift a gear box 144 whichprovides the ac.- tual output to the spindle.

Since this arrangement provides a very precise slide motion per spindlerevolution, it is well adapted to generate the lead for single pointcutting of threads. Commonly, cutting of threads requires more than onepass to realize the final depth. Each pass must start at the same pointin the groove so as to create the desired thread form. To accomplishthat a marker pulse generator 148 is associated with a spindle 40, anddetects' the rotation of a single point on'the spindle and provides amarker pulse which thus occurs once each rotation. Associatedelectronics 150 operate on this pulse and provide it to a gate 152. Whenthe thread mode store unit 82 contains a signal indicating that a threadis being cut, a second conditioning signal is provided to the gate 152and the marker pulse is provided to the gate 98, allowing the spindlepulses to be provided from the pulse transducer to the feedrateinterpolator 52. When the threading mode store is .off, the gate 98 isalways conditioned so that the feedrate pulses are not delayed. Withthis arrangement, when a thread is being cut, the generation ofinterpolator pulses is always initiated at the same point relative tothe rotation of the spindle.

In operation, after receiving a block of information of the typeillustrated in FIG. 4, the system uses the existing contents of thestore 116 and the contents of the SFM store 72 to generate a calculatedRPM. Based on that RPM, as long as it does not exceed a maximumgear-range RPM, a spindle speed number is calculated, and is used todrive the spindle. A train of spindle pulses is then generated by thetransducer 90 and is multiplied by the feedrate number which wascalculated by the system on the basis of stored delta X, delta Z and [PRnumbers, in the feedrate interpolator 52. The -add command pulse trainprovided by the feedrate interpolator is used to control the rate ofoperation of the interpolators 48 and 50, driving the X and Y axesrespectively. Motion of the X axis modifies the contents of the radiusstore 116 and thereby modifies the calculated RPM and the rate ofrotation of the spindle. The system thus operates to perform thecommanded motions at such rates as to realize the commanded SFM and IPRcommands.

Having thus described my invention, 1 claim:

1. A control system for. a lathe having a rotary spindle and a cutterpositionable with respect to said spindle, comprising: a source ofnumerical information relating to a desired path of motion of the cutterrelative to the spindle; a variable speed drive system responsive tosaid information for the spindle; a drive system for positioning thecutter relative to the spindle; means for generating a control signalwhich is a function of the distance of the cutter from the axis ofrotation of the spindle; and means for providing said control signal tosaid variable speed spindle drive so as to cause the drive speed to bemodifiedas a function of the said control signal. 7

2. The system of claim 1 wherein said control signal is directlyproportional to the distance between the center of rotation of thespindle and the cutter and said variable speed spindle drive iscontrolled in inverse proportion to said control signal, whereby therate of travel of the cutter over a workpiece supported on the spindleis. maintained constant, independent of the separation of the cutterfrom the axis of rotation of the spindle.

3. The system of claim 1 wherein said source of numerical informationincludes information relating to the desired rate of travel of thecutter over the surface of a workpiece supported on the spindle, andwherein said variable speed spindle drive is controlled as a directfunction of the value of said numerical information and as an inversefunction of the control signal.

4. The system of claim 1 wherein said control signal is directlyproportional to the distance of the cutter from the center of rotationof the spindle, the system includes a second source of numericalinformation relating to the desired rate of travel of the cutter over aworkpiece supported in the spindle, means are provided for deriving asecond control signal from said second numerical information, and saidvariable speed spindle drive is controlled as a direct function of thevalue of said second control signal, and as an inverse function of thecontrol signal which is a function of the distance of the cutter fromthe axis of rotation of the spindle.

5. The control system of claim 1 wherein said drive system forpositioning the cutter is controlled by signals derived from said sourceof numerical information so that the motion of the cutter along saiddesired path occurs at a rate proportional to the rate of rotation ofthe spindle.

6. The control system of claim 5 wherein said source of numericalinformation includes information relating.

to the rate of motion of the cutter and the drive system for positioningthe cutter is caused to move at a resultant rate which is a directfunction of said last said numerical information and a direct functionof the rate of rotation of the spindle.

7. The system of claim 6 wherein the numerical information relating tothe desired path of motion is expressed as a series of increments ofmotion and control signals for the drive system for positioning thecutter are generated, for each increment, at rates directly proportionalto both the rate of rotation of the spindle and the numerical value ofthe rate control signal.

8. A control system for a machine having a rotary spindle and a cutterpositionable with respect to said spindle, comprising: a source ofnumerical information relating to a desired path of motion of the cutterwith respect to the spindle, such motion being expressed as a series ofconnected motion segments, the numerical information includinginformation relating to the rate of travel of the cutter over aworkpiece supported on the spindle; means for generating a first controlsignal which is the function of the distance of the cutter from the axisof rotation of the spindle; a variable speed drive system for thespindle; means for controlling said variable speed drive system as adirect function of said numerical information relating to the rate ofmotion of the cutter over the workpiece supported on the spindle, and ininverse proportion to the said first control signal; means forgenerating a second control signal proportional to the rate of rotationof the spindle; and means for controlling the motion of the cutterrelative to the spindle in accordance'with said numerical informationand at a rate directly proportional to said second control si ns.

9. e system of claim 8 wherein the cutter lS movable along a pair ofaxes, one of which is normal to the axis of rotation of the spindle, andthe other of which is parallel to the axis of rotation of the spindle,the numerical information relating to each segment of motion includesincrements of motion along said two axes, interpolators are provided foreach of the axes to generate control signals at rates proportional tothe numerical information, and the interpolators are operated at ratesproportional to the second control signal.

10. The system of claim 9 wherein the second control signal consists ofa train of pulses generated by a transducer physically connected tospindle and operative to generate a pulse for each incremental rotationof the spindle.

1 1. The system of claim 10 wherein the interpolators operate torepeatedly add the numerical information relating to the motion segmentsalong each axis into registersand to generate output pulses as afunction of the overflow of said registers, and an addition is made intoeach of said registers each time a pulse occurs in -the train of pulsesgenerated by said transducer.

12. The system of claim 11 wherein the pulse train output of thetransducer is multiplied by numerical information relative to thedesired rate of motion of the cutter relative to the spindle axis todevelop a second pulse train which is used to control the rate ofoperation of the interpolators.

13. A control system for a machine having a rotary spindle and a cutterpositionable with respect to such spindle, comprising: a source ofnumerical information relating to the desired path of motion of thecutter relative to the spindle and the desired rate of travel of thecutter over the surface of a workpiece supported on the spindle; meansfor rotating the spindle at a rate which is a direct function of thecommanded rate of travel of the cutter over the surface of a workpiecesupported on the spindle and an inverse function of the normal distancebetween the cutter and the center of rotation of the spindle; and meansfor controlling the position of the cutter relative to the spindle so asto move the cutter through the path commanded by said numericalinformation at a rate proportional to the rate of rotation of thespindle.

14. The control system of claim 13 wherein the position of the cutterrelative to the spindle is controlled by means which includes at leastone digital servo operative to move the cutter along an axis normal tothe center of rotation of the spindle and the system includes means forgenerating a train of control pulses from said numerical information forpowering said digital servo, and a reversible register operative toreceive said train of pulses and to algebraically store them so as tomaintain a number proportional to the distance of the cutter from thecenter of rotation of the spindle along normal lines.

15. The control system of claim 14 wherein means are provided forpresetting the contents of said register with a number proportional tothe normal distance between the cutter and the center of rotation of thespindle prior to the initiation of utilization of a section of numericalinformation.

1. A control system for a lathe having a rotary spindle and a cutterpositionable with respect to said spindle, comprising: a source ofnumerical information relating to a desired path of motion of the cutterrelative to the spindle; a variable speed drive system responsive tosaid information for the spindle; a drive system for positioning thecutter relative to the spindle; means for generating a control signalwhich is a function of the distance of the cutter from the axis ofrotation of the spindle; and means for providing said control signal tosaid variable speed spindle drive so as to cause the drive speed to bemodified as a function of the said control signal.
 2. The system ofclaim 1 wherein said control signal is directly proportional to thedistance between the center of rotation of the spindle and the cutterand said variable speed spindle drive is controlled in inverseproportion to said control signal, whereby the rate of travel of thecutter over a workpiece supported on the spindle is maintained constant,independent of the separation of the cutter from the axis of rotation ofthe spindle.
 3. The system of claim 1 wherein said source of numericalinformation includes information relating to the desired rate of travelof the cutter over the surface of a workpiece supported on the spindle,and wherein said variable speed spindle drive is controlled as a directfunction of the value of said numerical information and as an inversefunction of the control signal.
 4. The system of claim 1 wherein saidcontrol signal is directly proportional to the distance of the cutterfrom the center of rotation of the spindle, the system includes a secondsource of numerical information relating to the desired rate of travelof the cutter over a workpiece supported in the spindle, means areprovided for deriving a second control signal from said second numericalinformation, and said variable speed spindle drive is controlled as adirect function of the value of said second control signal, and as aninverse function of the control signal which is a function of thedistance of the cutter from the axis of rotation of the spindle.
 5. Thecontrol system of claim 1 wherein said drive system for positioning thecutter is controlled by signals derived from said source of numericalinformation so that the motion of the cutter along said desired pathoccurs at a rate proportional to the rate of rotation of the spindle. 6.The control system of claim 5 wherein said source of numericalinformation includes information relating to the rate of motion of thecutter and the drive system for positioning the cutter is caused to moveat a resultant rate which is a direct function of said last saidnumerical information and a direct function of the rate of rotation ofthe spindle.
 7. The system of claim 6 wherein the numerical informationrelating to the desired path of motion is expressed as a series ofincrements of motion and control signals for the drive system forpositioning the cutter are generated, for each increment, at ratesdirectly proportional to both the rate of rotation of the spindle andthe numerical value of the rate control signal.
 8. A control system fora machine having a rotary spindle and a cutter positionable with respectto said spindle, comprising: a source of numerical information relatingto a desired path of motion of the cutter with respect to the spindle,such motion being expressed as a series of connected motion segments,the numerical information including information relating to the rate oftravel of the cutter over a workpiece supported on the spindle; meansfor generating a first control signal which is the function of thedistance of the cutter from the axis of rotation of the spindle; avariable speed drive system for the spindle; means for controlling saidvariable speed drive system as a direct function of said numericalinFormation relating to the rate of motion of the cutter over theworkpiece supported on the spindle, and in inverse proportion to thesaid first control signal; means for generating a second control signalproportional to the rate of rotation of the spindle; and means forcontrolling the motion of the cutter relative to the spindle inaccordance with said numerical information and at a rate directlyproportional to said second control signs.
 9. The system of claim 8wherein the cutter is movable along a pair of axes, one of which isnormal to the axis of rotation of the spindle, and the other of which isparallel to the axis of rotation of the spindle, the numericalinformation relating to each segment of motion includes increments ofmotion along said two axes, interpolators are provided for each of theaxes to generate control signals at rates proportional to the numericalinformation, and the interpolators are operated at rates proportional tothe second control signal.
 10. The system of claim 9 wherein the secondcontrol signal consists of a train of pulses generated by a transducerphysically connected to spindle and operative to generate a pulse foreach incremental rotation of the spindle.
 11. The system of claim 10wherein the interpolators operate to repeatedly add the numericalinformation relating to the motion segments along each axis intoregisters and to generate output pulses as a function of the overflow ofsaid registers, and an addition is made into each of said registers eachtime a pulse occurs in the train of pulses generated by said transducer.12. The system of claim 11 wherein the pulse train output of thetransducer is multiplied by numerical information relative to thedesired rate of motion of the cutter relative to the spindle axis todevelop a second pulse train which is used to control the rate ofoperation of the interpolators.
 13. A control system for a machinehaving a rotary spindle and a cutter positionable with respect to suchspindle, comprising: a source of numerical information relating to thedesired path of motion of the cutter relative to the spindle and thedesired rate of travel of the cutter over the surface of a workpiecesupported on the spindle; means for rotating the spindle at a rate whichis a direct function of the commanded rate of travel of the cutter overthe surface of a workpiece supported on the spindle and an inversefunction of the normal distance between the cutter and the center ofrotation of the spindle; and means for controlling the position of thecutter relative to the spindle so as to move the cutter through the pathcommanded by said numerical information at a rate proportional to therate of rotation of the spindle.
 14. The control system of claim 13wherein the position of the cutter relative to the spindle is controlledby means which includes at least one digital servo operative to move thecutter along an axis normal to the center of rotation of the spindle andthe system includes means for generating a train of control pulses fromsaid numerical information for powering said digital servo, and areversible register operative to receive said train of pulses and toalgebraically store them so as to maintain a number proportional to thedistance of the cutter from the center of rotation of the spindle alongnormal lines.
 15. The control system of claim 14 wherein means areprovided for presetting the contents of said register with a numberproportional to the normal distance between the cutter and the center ofrotation of the spindle prior to the initiation of utilization of asection of numerical information.